U.S. Democrats opened their national convention in Philadelphia Monday amid renewed tension between backers of nominee-in-waiting Hillary Clinton and former rival Bernie Sanders.

With many of his supporters angry about leaked emails from Democratic Party leaders that seemed to show a bias in favor of Hillary Clinton, Sanders sought to heal the party rift. He urged his loyalists to support nominee-in-waiting Hillary Clinton.

“I served with her in the United States Senate and know her as a fierce advocate for the rights of children, women and the disabled. Hillary Clinton will make an outstanding president, and I am proud to stand with her here tonight,” said Sanders.

Anger among Sanders supporters was visible outside the Wells Fargo Center, where hundreds of demonstrators were confronted by police, resulting in several arrests.

A supporters of Sen. Bernie Sanders, I-Vt., holds up a sign call calling for Debbie Wasserman Schultz, chairwoman of the Democratic National Committee to be fired, Sunday, July 24, 2016, in Philadelphia.

Sanders supporter Lisa LeVally Evans remained upset about the email revelations, saying “The media is in on it. Everybody was in on it. They said we were crazy. Well, look who's crazy now.”

But Sanders delegate Richard Cassidy from Vermont said Democrats will come together eventually because of Republican Donald Trump: “I think it will be a very small group of Sanders supporters who don't vote for Hillary Clinton, because the alternative is just unpalatable."

Among the opening night speakers was First Lady Michelle Obama, who also urged Democrats to support Clinton.

She said Republican nominee Donald Trump is wrong in his assessment of the state of the union: “Don't let anyone ever tell you that this country isn't great, that somehow we need to make it great again. Because this right now is the greatest country on earth.”

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Day 1: Democratic National Convention

Massachusetts Senator Elizabeth Warren also revved up the crowd with a forceful attack on Trump: “When we turn on each other rich guys like Trump can push through more tax breaks for themselves and we’ll never have enough money to support our schools or rebuild our highways or invest in our kids' futures.”

Clinton and vice presidential running mate Tim Kaine will both address the Democratic Convention later in the week.